Chemistry.. balancing equations?

Question

Ok. I'm seriously STUMPED on 4 questions. the numbers in the stars * * are the subscript things. Please help! I've done 36 others correctly, i just can't understand these ones!

1. (NH*4*)*2* SO*4* + KOH = K*2* SO*4* + NH*3* + H*2*O

2. HfCl*3* + Al = HfCl*2* + AlCl

3. BaCO*3* + C + H*2*O = CO + Ba(OH)*2*

4. CU +H*2*SO*4* = CuSO*4* + SO *2* + H*2*O

It's balancing equations. For some reason, i can't figure it out. Can anyone help me please?

1.

Answer 1

These really aren't hard -- the process is just like equations in algebra.  You just make sure that you have exactly the same number of atoms of each element on both sides in order to balance the equation.

Let's work through one so you can see the process:

1)  Ammonium sulfate and potassium hydroxide reacts to form potassium sulfate, ammonia gas, and water (in reality, they would all remain as ions in water, but let's just go with it for a moment):

Ammonium sulfate is (NH4)2SO4

the ammonium ion is the only source of nitrogen, so it MUST balance with the ammonia gas on the right side of the equation.

Potassium hydroxide (KOH) is the only source of potassium ions, so you need two KOH for every SO4

So, starting from there, let's count 'em up:

(NH4)2SO4 + 2 KOH:  two nitrogen, eight hydrogen, one sulfur, and four oxygen from the ammonium sulfate, plus two potassium, two more oxygen, and two more hydrogen from the potassium hydroxide, means a total of: two nitrogen, six hydrogen, one sulfur, two potassium and ten oxygen.

Now, how do I balance that on the right side?  The nitrogen must be as ammonia (NH3), so I must have two ammonia molecules.  The two potassium from KOH are already balanced in one molecule of K2SO4, so the equation is now:

(NH4)2SO4 + 2 KOH ==> K2SO4  + 2 NH3 + ? H2O

All I need to do now is make sure the hydrogen and oxygen are balanced, and for that I'll use the water.  I have two OH ions from the potassium hydroxide, and two excess hydrogen ions from the ammonium, which can combine to make two molecules of water, so:

(NH4)2SO4 + 2 KOH ==> K2SO4  + 2 NH3 + 2 H2O

Let's count 'em up again:

Left side:  two nitrogen, one sulfur, six oxygen, ten hydrogen, two potassium

Right side: two potassium, one sulfur, six oxygen, two nitrogen, ten hydrogen

and the equation is balanced!

Now, see how much progress you make with the other three examples.  Remember, the numbers of atoms must be equal on both sides!